Semiconductor lasers have found many industrial and commercial applications in recent years. For example, lasers are used in telecommunications, in pickups for optically readable media used in CD players, CD ROM drives and DVD players, in medical imaging, and in video displays. However, previously known semiconductor lasers have a number of disadvantages. For example, traditional semiconductor lasers, such as ones used in CD players, emit light from the edge of a chip, making it necessary to cleave a wafer into chips and to package the chip before determining whether the laser functions properly.
In recent years, chiral materials, such as cholesteric liquid crystals, have been demonstrated and proposed in a variety of lasing and filtering applications to address common drawbacks of standard semiconductor devices such as VCSELs. For example, a commonly assigned U.S. Pat. No. 6,404,789 entitled “Chiral Laser Apparatus and Method,” discloses a chiral laser with a defect formed by a light-emitting material layer. While this approach is advantageous with respect to previously known techniques, it may be difficult to construct a thin film structure having a precise light emitting material thickness required to produce a defect (the required thickness must be approximately equal to the wavelength of light in the medium divided by 4). More importantly, the position of the localized state caused by the defect cannot be easily controlled because the thickness of the light-emitting material cannot be changed once the device is manufactured.
One approach that addressed this problem was disclosed in the commonly assigned U.S. Pat. No. 6,396,859 entitled “Chiral Twist Laser and Filter Apparatus and Method” which is hereby incorporated by reference herein in its entirety. The novel approach of this patent involved creating a localized state by inducing a defect in a chiral structure composed of multiple chiral elements, by twisting one element of the chiral structure with respect to the other elements along a common longitudinal axis such that directors of the element's molecular layers that are in contact with one another are disposed at a particular “twist” angle therebetween. The resulting “chiral twist structure” enabled control of the position of the localized defect state within the photonic band gap by varying the twist angle.
This novel chiral twist structure is advantageous for a variety of applications including, but not limited to, EM filters, detectors, and lasers that are readily tunable by varying the twist angle. However, the chiral twist structure has some limitations with respect to chiral lasers. An efficient laser must maximize the energy absorption from the pump beam. This requires lasers with lengths of a fraction to several centimeters. Efficient lasing further requires that the region of high mode intensity (high local density of states) should overlap the gain region. The previously discussed abrupt chiral twist structure, has an region of high gain with length of the order of one or several localization lengths, which are comparable to the wavelength. However, this region is too short to efficiently absorb pump radiation since in such standard chiral twist structures, a short localization length is associated with the desirable features of long photon dwell time in the structure and wide photonic band gap. Another drawback of the chiral twist defect structure, is that the photon lifetime plateaus with increasing sample length, thereby limiting the reduction in the laser threshold that can be achieved with this structure. [See “Twist Defect in Chiral Photonic Structures,” V. I. Kopp and A. Z. Genack, Phys. Rev. Lett. 89, 033901 (2002)]. In addition, in chiral fiber structures produced by twisting glass fibers, it is difficult to achieve a sharp twist angle.
It would thus be desirable to provide a novel chiral defect structure that has a smoother variation of the energy distribution along an extended portion of its length and thus absorbs pump radiation more efficiently than a standard chiral twist structure. It would be further advantageous to provide a structure, which overcomes the limitation associated with the saturation of the lifetime of the mode and which can be readily manufactured in a continuous process.